Traditional composite structures for airplane industry are often made up of stiffened panels of monolithic shells or sandwich structures, assembled together with shell or panel stiffeners such as stringers. Composite prepreg material i.e. layer of fiber material preimpregnated with resin are commonly used in the manufacture of stiffenings. Composite prepregs are high in strength and light in weight, which makes them useful materials for aircraft structures, automobiles, ships, etc.
Today stringers including advanced integrated panel stiffeners such as hat-shaped, I-shaped and Z-shaped profiles are being more frequently used in stiffened panels of aircraft structures.
For example, the EP application No. 1149687B1 discloses a method for producing a body structure composed of a stiffened panel. The body structure is composed of a skin member, a plurality of stringer members arranged longitudinally of the body structure and a frame member arranged substantially perpendicular to the stringer members and intersecting with the stringer members. The frame member and stringer members are bond to the skin member. The frame member has a cylinder-shaped body structure or an elliptic cylinder-shaped body structure comprising a plurality of outer frame members arranged in a line which is intersected by the stringer members. Joggles provided in both edge portions of the preformed outer frame members are integrated with the preformed stringer members. Furthermore, the inner frame member is bond to the outer frame members to span the stringer members and to form a continuous frame member.
In general the manufacturing of structures having integrated panel stiffening requires many components and operations. Problems with the tolerance of different components may lead to time-consuming shimming operations. Furthermore, the assembly of stiffeners on curved shell structures is of the time consuming as it may require manual operations. These facts may lead to expensive and time consuming manufacturing processes.
For these reasons stiffening elements and structures comprising stiffening elements may be subjected to development and further improvement.